Collation Shrink Wrap Process
Beverage bottles have been incorporated into packs by shrinking a collation shrink film using heat around a number of bottles, so bundling the bottles together. Such packaging processes are commonly referred to as collation shrink packaging processes. Such processes are distinct from other bundling procedures such as wrapping items using stretch cling films or by elastically stretching a stretch hood tubular film around the items to be bundled at ambient temperature.
Collation shrink films are conventionally produced using a blown film extrusion process. The film is stretched in a molten condition in the direction of the film take up longitudinally (Machine Direction, MD stretch) at a specified draw down ratio (DDR) and transversely thereto by inflation of the extruded bubble of film (Transverse Direction, TD stretch) at a specified blow up ratio (BUR). These ratios are determined relative to the circumference and longitudinal speed of the molten film emerging from the extruder. The solidified tubular film is flattened and taken up on a roller. Typically, the tube is slit and the flat film is wound on two separate winders creating two reels of flat film. This film is then generally printed. Collation shrink films may be coextruded to form a multi-layer film structure.
During the collation shrink packaging process, the film is wrapped around the items to be bundled or packaged. Where the film edges overlap, the weight of the packaged items combined with heat used to create shrinkage fuses overlapping edges of the film together. Heat is applied to shrink the film to cause a partial reversion of the stretch. On completion, the film holds the packaged material tightly for shipping.
Generally, in coextruded multi-layer collation shrink films, shrinkage takes place in a central layer containing low density polyethylene (LDPE) with outer layers configured not so much to provide shrinkage but to provide puncture resistance and optical properties. The outer layers may contain linear low density polyethylene (LLDPE) which generally does not contribute to shrinkage.
Types of Polyethylene
LDPE is defined herein as a low density polyethylene having a density of from 0.915 to 0.935 g/cm3. LDPE is often highly branched. The branching leads to an elevated shear sensitivity reflected in the melt index ratio (I21.6/I2.16) at 190° C. which ranges from 45 to 100. The melt index ratio (MIR), which is a measure of shear sensitivity, is expressed as a ratio of the melt index of the polymer determined according to ASTM D-1238, condition F, 21.6 kg at 190° C., divided by the melt index of the polymer determined according to ASTM D-1238, condition E, 2.16 kg at 190° C. The melt index ratio may be referred to herein as MIR I21.6/I2.16 at 190° C., or simply as MIR.
A further measure of the nature of the branching is provided by the Melt Strength Factor (MSF), the determination of which is described herein. Because of the general presence of long chain branching, LDPE's may have an MSF of at least 0.01 Newton.
LDPE's are typically produced using free radical initiation at high pressure and temperature and hence have a broad molecular weight distribution Mw/Mn of at least 5, as determined by GPC using a differential refractive index (DRI) detector and low angle laser light scattering (LALLS) measurements as described, for example, in WO 2010/47709-A1, incorporated herein by reference.
LLDPE is defined herein as having a density of from 0.915 g/cm3 to 0.940 g/cm3. The reduced density is often obtained by copolymerizing ethylene with an alpha-olefin comonomer having from 4 to 10 carbon atoms using a transition metal based catalyst system.
Certain types of LLDPE's are highly linear and lack long chain branching. This is reflected in a reduced shear sensitivity compared to LDPE. LLDPE's typically have a MIR from 16 to 40. Highly linear LLDPE's typically have an MSF of less than 0.003 Newton. Such linear LLDPE's also have a low relaxation time as determined though the Cross model, as described later herein, of less than 1 or 0.5 second.
Other types of LLDPE have some long chain branching obtained through the use of certain transition metal catalysts during the polymerization process, such as certain metallocenes. Such branched LLDPE's generally have an MSF of less than 0.01 Newton and/or more than 0.003 Newton, preferably more than 0.004 Newton, and more preferably more than 0.005 Newton, and show higher relaxation times as determined through the Cross model of at least 0.5 or 1 second.
Other forms of generally linear polyethylenes may be high density polyethylene (HDPE) having a density in excess of 0.940 g/cm3 and a very low density linear polyethylene (VLDLPE) having a density of less than 0.915 g/cm3. VLDLPE and LLDPE's generally have a narrow Mw/Mn of less than 5. The extent of long chain branching may be reflected inter alia in the MIR values and MSF values.
VLDLPEs may have a modest degree of long chain branching when they are produced using selected metallocene transition metal based catalyst systems and suitable process conditions during the polymerization process. Overall VLDLPE, LLDPE and HDPE have reduced long chain branching as compared to LDPE.
Prior Art Discussion
A multi-layer film with LLDPE outer layers and an LDPE core layer is described in U.S. Pat. No. 4,657,811 where the foamed core layer is prepared using an azodicarbonamide blowing agent, which serves to stiffen the film. The preparation of azodicarbonamide foaming agent masterbatches is disclosed in U.S. Pat. No. 8,158,690. There is no suggestion that the film be adapted to be flexible and be used for collation shrink application in which the shrinkage of outer LLDPE layers is utilized to thicken and increase the thermal insulation provided by the film.
A polyethylene resin foam sheet suitable for shock absorbing packaging applications is disclosed in EP 2708344-A1. The sheet is prepared in stages using coextrusion of a polyethylene foam layer followed by lamination with an oriented polypropylene (OPP) film. EP 2708344 A1 does not mention orientation of the film or shrinkage properties that would result from such orientation. The outer layer of OPP makes it highly unsuitable for collation shrink, even assuming that other film properties were to be selected for such a purpose, which is not discussed in the document. It is not suggested that the laminate can be used for collation shrink application with an attendant improvement of the thermal insulation resulting from the collation shrink step.
The film in EP 2653391A1 is said to be optimized for collation shrink applications after blown film extrusion by uniaxially orienting it in the machine direction 3 to 10 times. This results in a considerable down-gauging (i.e. thinning) of the film, and hence a reduction in the thermal insulation the film provides. This does permit use of a lower temperature to induce the shrinkage. The films are not stretched in the transverse direction. A bundling force in the transverse direction has to be provided separately by a tape. The underlying concept of EP 2653391-A1 is incompatible with the notion of collation shrinking a biaxially oriented film with an inner layer containing foamed bubbles so as to create a thicker thermally insulating film in the very collation shrinking process.
Furthermore collation shrink coextruded films with layers containing HDPE are described in EP 1529633-A1. Also collation shrink coextruded films with an LDPE layer sandwiched between conventional LLDPE layers are described in U.S. Pat. No. 6,187,397. Coextruded films with improved shrink properties using blends of LDPE with metallocene-derived polyethylenes are described in WO 2001/44365-A1. US 2009/0110913-A1 describes coextruded structures for broad range of packaging application using long chain branched LLDPE polymers.
Monolayer or coextruded films for heat shrink applications using a long chain branched LLDPE made using a suitable metallocene based catalysts system are described in WO 2004/22646-A1. The LLDPE may have an MIR (I21.6/I2.16) at 190° C. of from 30 to 80. U.S. Pat. No. 6,255,426 describes such polymers. In coextruded structures such LLDPE materials may be combined with LDPE layers. Similar structures are described in US 2012/0100356-A1. Collation shrink is mentioned as a possible application in WO 2009/109367-A1 which further describes high MIR LLDPE's produced using suitable metallocene based catalyst systems.
Outside of the field of collation shrink, foamed films or sheets have been described for example in US 2008/0138593, U.S. Pat. No. 7,341,683 and US 2012/02288793.
It is among the aims of the invention to provide procedures and films through which finely dispersed bubbles in the polymer matrix of a coextruded film layer can be used to increase thermal insulation of the packaged items and/or improve physical protection of the packaged items, while preserving other desirable package characteristics such as puncture resistance for safe handling of a bottle pack.